Protection of in-stream biota from urban impacts: minimise catchment imperviousness or improve drainage design?

Urbanisation is a looming global threat to in-stream biodiversity, but the best approaches to mitigation are unclear. This paper asks if the protection of in-stream biota, in particular macroinvertebrate assemblages, is dependent on the sequestration of catchments from urbanisation, or if protection in urbanised catchments can be achieved through better drainage design. In-stream macroinvertebrate assemblage composition was assessed for 16 catchments spanning a rural–urban gradient. Catchment imperviousness and drainage connection (the proportion of impervious area directly connected to streams by stormwater pipes), together with other possible driving factors, were assessed as explanatory variables of macroinvertebrate assemblage composition. The proportion of variance explained independently and jointly by each variable was assessed by hierarchical partitioning. Assemblage composition was strongly explained by the gradient of urban density (i.e. a large proportion of variance was jointly explained by variables correlated with the urban gradient; imperviousness, connection, longitude and elevation). However, drainage connection was the strongest independent correlate. Most sensitive taxa were absent from sites with >20% connection. Thus the connection of impervious surfaces to streams by pipes is a more likely determinant of taxa loss than the impervious areas themselves. Low-impact urban design approaches that reduce drainage connection are postulated as the most effective management solution to the protection of stream biota in urban catchments.